In a variety of fields such as cosmetics, household care compositions and mold release agents, there is a desire for high-viscosity organopolysiloxanes to be rendered into fine emulsions. However, when high-viscosity organopolysiloxanes are directly emulsified, the size of the emulsion particles has a lower limit on the order of several microns; obtaining emulsions finer than this has been difficult. Accordingly, various methods for producing emulsions by emulsion polymerization have been investigated in order to obtain fine emulsion particles.
For example, methods for carrying out emulsion polymerization on a cyclic siloxane oligomer in an emulsified state by using a strong acid or a strong base are known (see, for example, Patent Documents 1 and 2). Using such methods, it is possible to obtain an emulsion having an emulsion particle size of 300 nm or less.
Of related interest, methods for the production of, by emulsion polymerization, stable emulsions containing high-viscosity organopolysiloxanes capped with trialkylsilyl groups have been investigated. For example, a method for producing a high-viscosity organopolysiloxane capped with trialkylsilyl groups in the presence of a hydroxy-terminated organopolysiloxane, a cyclic organopolysiloxane and a trialkyl-terminated organopolysiloxane having a viscosity at 25° C. of 50 mm2/s or less has been investigated (Patent Document 3). In addition, a method of production in the presence of a hydroxy-terminated organopolysiloxane and a trialkyl-terminated organopolysiloxane having a viscosity at 25° C. of at least 55 mm2/s has been investigated (Patent Document 4). However, under the above conditions, because trialkylsilyl end groups are present at the initial stage of polymerization, chain transfer and chain termination arise in parallel, and so the rate of polymerization is slower than in a system containing only a hydroxy-terminated organopolysiloxane. As a result, because setting the organopolysiloxane included in the emulsion to the desired viscosity takes time, one drawback is that octamethylcyclotetrasiloxane tends to be produced within the organopolysiloxane included in the resulting emulsion.
Therefore, setting the trialkyl-terminated organopolysiloxane within an emulsion to a desired viscosity in as short an emulsion polymerization time as possible is strongly desired not only for greater production efficiency, but also to suppress the octamethylcyclotetrasiloxane included in the organopolysiloxane within the emulsion.